Genomes can be altered in a variety of ways including single base pair changes, deletions and duplications of sequences, and chromosome rearrangements (translocations and inversions). The rates at which these events occur affect 2 quite different processes. Genetic changes are required for evolution and the rate of these changes, therefore, can in principle affect the rate of evolution. Second, many cancers are associated with elevated rates of chromosome rearrangements, and this type of instability appears to be strongly associated with metastatic tumors. When mammalian cells are treated with aphidicolin (a DNA polymerase inhibitor), the chromosomes are broken at specific sites (fragile sites) that are related to translocation breakpoints observed in tumor cells. High levels of chromosome breaks and other aberrations are also observed in mammalian cells with mutations in the ATM and ATR genes. Our general goal is to mimic this type of genetic instability in the yeast Saccharomyces cerevisae, and to characterize the mechanisms responsible for this instability. Our specific aims are: 1) to determine the DNA sequence properties that characterize yeast fragile sites and to characterize the chromosome rearrangements that are associated with DNA breaks at fragile sites, 2) to characterize chromosome aberrations in yeast strains with mutations in genes affecting DNA damage checkpoints, telomere length regulation, recombination, and/or chromatin assembly;for example, strains with mec1 and/or tel1 genes (yeast homologues of the mammalian ATR and ATM genes, respectively) will be examined, and 3) to understand the genetic instability induced by ionizing radiation. These studies will emphasize techniques that allow examination of the entire yeast genome, including DNA microarray analysis (to detect changes in gene dosage and to map translocation breakpoints), and gel technologies that allow separation of intact chromosomal DNA molecules.